Last summer I built a big patio table. Over the past year, it has changed in a way which surprises me, and which I can't explain.
The table is made from recycled old-growth redwood. It is 12 feet long. The top is 2 1/2" thick. It is supported by two trestles which are about twenty inches from the ends. That is, the main unsupported span is about eight feet. Despite that length, the table is rock solid. I can jump on it, and there's no flex. I "finished" the table with several coats of deck oil.
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When the table went into service last summer, the top was flat. It stood on the patio out in the weather over the winter. ("Winter", here, means about 15 inches of rain spread over four months, and a few days with morning frost.) Today, the table top is not flat. It sags about an inch in the middle. It is still rock solid.
I'm surprised at the sag. I don't have a lot of experience building outside furniture, but I've seen wood tables at park campsites that are fairly flat despite decades of exposure to the weather. So what has happened to my table? And how do I fix it?